In recent years, a demand for a secondary battery has spread not only to a market for mobile electronic devices but also to markets for large transporters such as electric vehicles and plug-in hybrid vehicles, emergency storage batteries for home use, and the like. While a lithium ion secondary battery is generally considered to be promising for these applications, a sodium-sulfur battery (commonly referred to as NAS battery) is being introduced in power generation plants or factories as a power storage unit requiring high-power by virtue of advantages in raw material cost and running cost.
The NAS battery is operated at high temperature (from about 300 to 350° C.) in order to keep sodium and sulfur serving as active materials in molten states and enhance ion conductivity of a β-alumina electrolyte. The molten sodium at an anode is oxidized to Na+ at an interface with β-alumina and moves to a cathode through the electrolyte. On the other hand, at the cathode, Na+ is reduced to sodium pentasulfide (Na2S5) by sulfur. The cell reactions described above (discharge reaction) can be represented by the following formulae. Herein, the upper formula (1) represents the reaction at the anode, the middle formula (2) represents the reaction at the cathode, and the lower formula (3) represents the whole reaction.2Na→2Na++2e−  (1)5S+2Na++2e−→Na2S5  (2)2Na+5S→Na2S5  (3)
The NAS battery is compact because of having a volume and weight about one-third of those of an existing lead storage battery. Therefore, the NAS battery can exhibit the same function as that of power generation with pumped-up water and can be placed near a place of demand such as an urban area. In addition, it is possible to combine the NAS battery with wind power generation or solar power generation having large output power variation to stabilize the output power. Moreover, it is possible to place the NAS battery in commercial-scale utility consumers such as plants or factories and charge the battery by utilizing cheaper night-time power, and at the same time, use the battery as an emergency power source in case of power outage. Further, the NAS battery has various advantages in that constituent materials are abundant and long-life resources, self-discharge is small, charge and discharge efficiency is high, the cost is expected to be reduced by mass production, and the like.
However, the NAS battery does not operate at normal temperature, and hence, there is a need to maintain the temperature in an operating temperature range (about 300° C.) by heating with a heater and using heat generated through discharge. Concerning the charge and discharge performance, the hour rate is set relatively longer (6 to 7 hours). In addition, there is a need for full charge reset within a certain period of time at present. Further, one problem that is difficult to solve practically is that when a fire accident is caused, aqueous fire-extinguishing chemicals generally used cannot be used because they react with metal sodium. Therefore, it is difficult for general fire departments to immediately respond to the fire and the applications and scale of cell capacity to be placed are significantly limited in the current situation.
In this connection, heretofore, a sodium ion secondary battery having more excellent safety has been proposed. In general, there is proposed a material prepared by replacing with sodium a lithium site of a material used for a lithium ion battery. Sodium has an ionic radius 30% larger than and a weight heavier than those of a lithium ion, and hence diffusion in the material is reduced as compared to the lithium ion. Therefore, the cathode, anode, and electrolyte are required to have a large free space that can be occupied by the alkali ion. For a cathode active material, several candidate materials have been found as disclosed in Non Patent Literatures 1 and 2 and Patent Literatures 1 and 2.
However, for example, NaCrO2 or the like disclosed in Non Patent Literature 1 has a layered rock salt structure and therefore is liable to deteriorate like lithium cobaltate, which offers a problem in structural stability.
Moreover, NaFePO4 disclosed in Patent Literature 1, which has the same composition as a phosphate material, LiFePO4, has not an olivine structure but of a maricite structure. Therefore, diffusion of the alkali ion is small (that is, the sodium ion is hard to move) and there are problems in structural stability and cycle performance practically.
Further, while Na2/3Fe1/3Mn2/3O2 disclosed in Patent Literature 2 and Non Patent Literature 2 is one having improved cycle performance by modifying the composition ratio of NaCrO2, there is a problem in structural stability, because of having a layered rock salt structure as with the substance disclosed in Patent Literature 1 described above.